This invention relates to polymer electrolyte fuel cells. More particularly, the invention relates to a method of manufacturing a separator built in a cell of a polymer electrolyte fuel cell.
A polymer electrolyte fuel cell (hereafter called a xe2x80x9cfuel cellxe2x80x9d) is an apparatus for power generation by supplying a reactant gas (hydrogen and oxygen) to an electrode which comprises a polymer electrolyte membrane. FIG. 3 is a perspective view of typical a cell C which is a minimum unit composing such a fuel cell. The cell C of a fuel cell comprises electrodes E1 and E2 (anode and cathode) which comprise a catalytic layer and a porous supporting layer, an electrolyte D inserted between the electrodes E1 and E2, and separators 100 disposed outside the electrodes E1, E2. Because as low as the voltage of 1 volt or less is obtainable on one cell C of the above-mentioned composition, practically speaking, tens to hundreds of those cells C are usually accumulated in series to form an actual fuel cell.
FIG. 4 is a front elevation of a conventional separator 100 used in a fuel cell. A number of grooves 120 of about 0.5-2.0 mm in width and depth are formed as shown in FIG. 4 on both sides of a plate-shaped separator 100. Those grooves 120 function as passages of the reactant gas and as exhaust passages of water generated as result of the reaction. In a fuel cell composed by a number of cells C accumulated as above, said separators 100 not only function as partitions of those cells C, but also function to supply the reactant gas to the adjacent electrode E1 (or E2) through the grooves 120 or to exhaust outside the water generated with the reaction. Moreover, the separators 100 play the role to transmit the electricity generated in the cells C outside. Therefore, in the separators 100 of a fuel cell, it is required that the gas shielding property be high so that the reactant gas supplied to the electrodes E1 and E2 (anode side and cathode side) should not mix with each other. Moreover, it is necessary to have excellent corrosion resistance and oxidation resistance so that it is never corroded with the reactant gas. In addition, it is also necessary for the fuel cell to be light, and to have the electrical conduction property. In addition, it is necessary that the separators have sufficient strength to bear the weight of the accumulated cells C.
At the same time, in order to minimize a fuel cell, it is necessary to make the thickness of the separator as thin as possible. An isotropic carbon is used as a material of the separator C which meets the above-mentioned requirements.
As shown in FIG. 5 schematically, in order to make a separator C by using an isotropic carbon, the following steps are performed; namely, firstly, a carbon material R is heated and sintered at 2000xc2x0 C. or more in an electric furnace (see FIG. 5(a)); secondly, it is cut out in the form of a plate (see FIG. 5(b)); and then grooves are mechanically formed with an end mill, etc. (see FIG. 5(c)). However, isotropic carbon after sintering is very hard and brittle, and consequently, there is a problem in the present art that carrying out the cutting and formation of grooves is too time-consuming.
Thus, there is another method in which the sintering material is prepared by mixing carbon powder with granulated phenol resin functioning as a binder, which material is charged into a mold formed with grooves. The components are sintered with a hot plate pressing. According to this method, it is advantageous that the sintering of the carbon powder and the formation of grooves may be accomplished simultaneously. However, in case of this method, because water is generated from phenol resin during the process of heating and sintering, bubbles which originate in water are inevitably formed in the carbon material after the sintering, thereby impairing the gas shielding property. Accordingly, in this method, there is a problem of having to give a processing of blocking the bubbles after the sintering, which again is very time-consuming.
There is still another method in which a metallic plate is arranged in a mold which is provided with grooves, and thereafter a sintering material comprising carbon powder and granulated phenol resin is charged thereinto. The components are sintered by a hot plate pressing, thereby integrating the metallic plate with the carbon material. A separator manufactured by this method has a structure in which the metallic plate is sandwiched by the carbon materials. Therefore, even if bubbles originating in the phenol resin in the carbon material are generated after the sintering, the obverse side and the reverse side of the separator are not communicable owing to the metallic plate installed in the center, thereby maintaining the gas shielding property as a whole.
However, in case of the prior art methods, there is a problem of generating oxides with a low electrical conduction property on the surfaces of the carbon material after the sintering depending upon the kinds of the metallic plate. Due to the difference in the coefficient of thermal expansion of the metallic plate and the carbon powder, there is a second problem in that the bonding strength in the interface of the metallic plate and the carbon material is rather weak, thereby giving rise to a premature separation.
In addition, because the sintering material is a mixture of the carbon powder and the granulated phenol resin, the composition tends to become ununiform. It is therefore necessary to add the granulated phenol resin in more than the necessary amount as a binder. Consequently, a number of bubbles which come from the phenol resin are generated. Also, the thickness of the carbon material after the sintering tends to be relatively large, and the electric resistance becomes larger thereby. Moreover, the quality of the carbon material after the sintering deteriorates if the sintering material is not uniform. Depending upon the shape of the grooves, the separability from the mold might become difficult, making it hard to remove the carbon material after sintering from the mold.
It would be of great advantage in the art if an improved method of manufacturing a separator for an electrolyte fuel cell.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. The present invention has firstly solved the above-mentioned problems by a method of manufacturing a separator for a polymer electrolyte fuel cell comprising the steps of: arranging a metallic plate provided with plating on the obverse and reverse sides thereof in a mold formed with grooves; charging a sintering material comprising a powder of carbon coated with phenol resin onto both sides of said metallic plate; and integrating said sintering material with said metallic plate by heating and sintering said sintering material in the atmosphere, thereby forming grooves on the surfaces.
The present invention includes a method of manufacturing a separator for a polymer electrolyte fuel cell comprising the steps of arranging a metallic plate provided with plating on the obverse and reverse sides thereof in a mold formed with grooves; charging a sintering material comprising a powder of carbon coated with phenol resin onto both sides of said metallic plate; and integrating said sintering material with said metallic plate by heating and sintering said sintering material in vacuum, thereby forming grooves on the surfaces.
The invention further includes a method of manufacturing a separator for a polymer electrolyte fuel cell comprising the steps of: preparing a mold formed with grooves; charging a sintering material comprising a powder of carbon coated with phenol resin a metal powder, and the same kind of sintering material as mentioned first into said mold in said order; and integrating said sintering materials with said metal powder by heating and sintering said sintering materials in vacuum, thereby forming grooves on the surfaces.